produce abundant agro-industrial residues such asbrewer’sspent grain (BSG), which are underexploited. BSG as the main by-product of brewing industry,representing approximately 85% of total by-products generated,

is rich in cellulose and non-cellulosicpolysaccharides and has a strong potential to be recycled. Due to the global intense pressure towardsgreen environmental technology, both academic and industrial researchers are putting more efforts toreduce the amount of such wastes by finding alternative uses apart from the current

general use as ananimal feed.

Thus, several products are increasingly being sought from BSG. This article intends toreview some of the products that can be realized from BSG and also tostimulate researchers to explorefurther,

especially in developing new value-added products.

Key

words:Brewer’s spent grain, polysaccharide, animal feed, residue.

INTRODUCTION

Beer is the fifth most consumed beverage in the worldapart from tea, carbonates, milk and coffee with an esti-mated annual world production exceeding 1.34 billionhectolitres in 2002 (Fillaudeau

et al., 2006).

In themanufacture of beer, various residues and by-productsare generated. The most common ones are spent grains,spent hops and surplus yeast, which are generated fromthe main raw materials (Mussatto, 2009).

Spent grains

are the by-products of mashing process;which is one of the initial operations in brewery in order tosolubilize the malt and cereal grains to ensure adequateextraction of the wort (water with extracted matter)(Fillaudeau

et al., 2006). Following different separationstrategies, the amount ofbrewers’spent grain (BSG)generated could be about

85% of the total by-products(Tang et al., 2009), which accounts

for 30 to 60% of thebiochemical oxygen demand (BOD) and suspendedsolids generated by a typical brewery (Hang et al., 1975).

It was reported that about 3.4 million tonnes of BSG fromthe brewing industry are produced in the EU every year(Stojceska et al.,2008), out of

over 0.5 million tonnes of thiswaste annually. However,Brazil,the world’s fourth largest beer producer

(8.5 billionlitres/year)

in 2002, generated ~1.7 million tonnes of BSG(Mussatto et al., 2006).

Thus, BSG is a readily available, high volume low costby-product of brewing and is a potentially valuableresource for industrialexploitation (Robertson

et al.,2010). Thus, increased endogenous metabolism as wellas high proteolytic activity in BSG affects its compositionwithin a very short time (Ikurior, 1995).

Several attempts have been made in utilizing BSG inanimal feeds, production of value-added compounds(xylitol, lactic acid, among others), microorganisms cul-tivation, or simply as raw material for extraction of com-pounds such as sugars, proteins, acids and antioxidants.It was also found to be applicable in enzymes production,as adsorbent for removing organic materials fromeffluents and immobilization of various substances(Mussatto, 2009).

This review

describes the feasibility oftransforming the BSG into different value

added productsto ensure a sustainable reuse of its bioresources.

PROXIMATE COMPOSITION OF BSG

Brewers’ spent grains are of high nutritive value (Tang

et

Aliyu and Bala 325

Table 1.

Chemical composition of brewers’ spent grain (BSG) as reported in the literature.

Components

(% dry weight)

Kanauchietal. (2001)

Russ et al.

(2005)

Mussatto and

Roberto (2006)

Mussattoet

al. (2008a)

Adeniranet

al. (2008)

Khidziret

al. (2010)

Cellulose

25.4

23-25

16.8

16.8± 0.8

-

-

Hemicellulose

-

30-35

28.4

28.4 ±2.0

-

-

Lignin

11.9

7.0-8

27.8

27.8 ±0.3

-

-

Proteins

24

19-23

15.3

-

2.4 ±0.2

6.4±0.3

Ashes

2.4

4-4.5

4.6

4.6 ±0.2

7.9±0.1

2.3±0.8

Extractives

-

-

5.8

-

-

-

Others

21.8**

-

-

22.4 ±1.2*

-

-

Carbohydrates

-

-

-

-

79.9 ±0.6

-

Crude fiber

-

-

-

-

3.3 ±0.1

-

Moisture contents

-

-

-

-

6.4± 0.2

-

Lipid

10.6

-

-

-

-

2.5±0.1

Acid detergent fibre

-

-

-

-

-

23.3

Total carbon (%)

-

-

-

-

-

35.6±0.3

Total nitrogen (%)

-

-

-

-

-

1.025±0.05

** Represents arabinoxylan and * stands for the combination of proteins and extractives.

al., 2009),andcontain cellulose,hemicelluloses, lignin

and high protein content as represented in Table1.

More-over, it is estimated that the annual production of plantbiomass in nature, of which over 90% is lignocellulose,amounts to about 200×109

variation in percentage compo-sition ofthe componentsis attributable to the variety ofthe grains used, harvest time, malting and mashingconditions, and the quality and type of

adjunctsusedduring

theprocess

(Robertson

et al., 2010).

TECHNIQUES FOR BSG PRESERVATION ANDSTORAGE

Several methods have been proposed to prolongbrewer’s spent grain (BSG)storage time as

a result of itshigh moisture content. Factory drying has been the mosteffective method of preserving BSG. However, owing tothe growing global concern over high energy cost,

manybreweries, especially those in the developing countriescan no longer afford this practice (Ikurior, 1995). Dryingas a preservation method has the advantage of reducingthe product volume, and decreases transport and storagecosts. Many breweries have plants for BSG processingusing two-step drying technique, where the water contentis first reduced to less than 60% by pressing, followed bydrying to ensure the moisture content is below 10%(Santos et al., 2003).

However, the traditional process for drying BSG isbased on the use of direct rotary-drum driers.

This pro-cedure isconsidered to be energy-intensive. Bartolome´et al. (2002) studied the effects of BSG preservationusing freeze-drying, oven drying and freezing methods.Their findings showed that preservation by oven drying orfreeze-drying reduces the volume of the product anddoes not alter its composition, while freezing is in-appropriate as it affects the composition of some sugarssuch as arabinose. But overall, freeze-drying is econo-mically not feasible at the large scale; making the oven-drying to be the preferred method.

Thin-layer drying using superheated steam wasproposed by Tang

et al.

(2005) as an alternative method.The circulation of superheated steam occurred in aclosed-loop system; this reduces the energy wastage thatoccurs with hot-air drying. Also, the exhaust steamproduced from the evaporation of moisture from the BSGcan be used in other operations. Thus, superheatedsteam method has several advantages including thereduction in the environmental impact, an improvement indrying efficiency, the elimination of fire or explosion risk,and a recovery of valuable volatile organic compounds.Another method is the use

to reachmoisture levels of between 20 and 30% (El-Shafey et al.,2004).

Moreover, chemical preservatives such aslactic,formic, acetic, benzoic acid and potassium sorbate caneffectively be used for preserving the quality andnutritional value of BSG as reported by Al-Hadithi et al.(1985).

SUSTAINABLE UTILIZATION OF BREWER’S SPENTGRAIN

Lignocellulosic substrates, being cheap and readilyavailable, have recently gained considerable interestbecause of their possible use in secondary fermentationprocesses. However, the utilization

ofBSG

is limited

especially in developing countries

and new ways ofmaking use of this residue would be beneficial for theprocess economy. The

following uses in various fieldshave been reported

in this

literature:

Animal nutrition andfeedformulations

The utilization of this abundantly available

raw material

has found a place inanimal

nutrition, which not onlyreduces the cost of feeding but also creates an outlet forthis material.

Thus,breweryspent

grains have beenutilized as feed foranimals

for many years

(Szponar

etal., 2003);

the

presenceof cellulose, hemicellulose andlignin, and also the amount of readily

available substan-cessuch as sugars and amino acidsaid

in its utilizationasfeed for ruminants

(Bisaria et al.,1997).

However,highmoisture content of BSG

(80

to85%)

together with polysaccharide and protein makes it parti-cularly susceptible to microbial growth and subsequentspoilageina short

intestinal digestion,alleviating both constipation and diarrhoea. Such effectswere attributed to the content of glutamine-rich protein,and to the high content of non-cellulosic polysaccharidesand smaller amounts of β-glucans (Tang

et al., 2009).

It was suggested that addition of ruminantly undegrad-able protein (RUP) to diets for lactating cows increasedmilk yield. The limiting amino acids associated withthis

utilizedforhumans,it can be incorporated into so many humandiets, such as breads and snacks;especiallywhere thereis need to boost the fibre contents. This may provide anumber of benefits; as dietary fibres have been reportedto aid in the prevention of certain diseases includingcancer, gastrointestinal disorders, diabetics and coronaryheart disease(Stojceska

et al., 2008).Increase in fibrecontent (from 2.3

to 11.5%) was observed when 30%BSG was incorporated into wheat flour for the productionof high-fiber enriched breads. However,degree ofsoftening and loaf volume were lower than

sawdust, tobaccoresidues, grass and processed tea waste (Demir, 2008,2006),have been used in building bricks

development, soas

to

reduce brick weight and increase its thermalinsulation ability

especially when considering the

recenttechnology of

green

building.

The low amount of ashcoupled with the high amount of fibrous material (lignin,hemicellulose and cellulose) makes

BSG suitable forusein building materials. Russ

et

al.

(2005)

found that firedfinished bricks produced with BSG have a characteristichigher strength, higher porosity (higher water absorptioncapacity) and a lower density, which give them betterproperties of thermal insulation than those produced froma similar production clay.

Thus, as a possible utilizationoption, agro-wastes generally being combustible, andduring the

firing process in the kiln, they can burn awayleaving pores in the brick.

Most frequently used poreformers in clay brick manufacturing have been classifiedeither as organic or inorganic. Thus, BSG and other agro-wastescan be considered as

organic pore-formingmaterials, which have the advantage of ensuring a heatcontribution to the firing furnace (Demir, 2008),reinforcing the structure during drying and counteractingcracking (Ducman and Kopar, 2001).

Metal adsorption andimmobilization

Several methods are used for removal of heavy metalsfrom wastewater. Butadsorption

The polysaccharide, protein content and high moisturecontents of BSG make it particularly susceptible tomicrobial growth and degradation. The presence of resi-dent microflora initiates these processes within theshortest time, in an attempt to utilize it as sole carbonsource (Robertson et al., 2010).

BSG

was reported to be used for the cultivation ofBifidobacterium adolescentis94BIM,Lactobacillussp.

In order for the microorganisms to grow on this residue,they produce a number of enzymes that aid in itsutilization such as endoxylanases, β-xylosidases, α-arabinofuranosidases and esterases (Mandalari

et al.,2008).However,

the substrate composition as well as thestrain used determines the enzyme type and activity. Thepresence of digestible and non-digestible organic resi-duesmakes BSG, a potential substrates on whichamylolytic organisms could be cultured for the productionof β-amylase and amyloglucosidase (Adeniran et al.,2008). Other enzymes of interest include xylanases,feruloyl

(Panagiotou et al., 2006). Com-plete breakdown of these components by micro-organisms requires the action of several enzymes whicharerecognized as axylanolytic

system/complex (Terrasanet al., 2010). Feruloyl esterases act synergistically withxylanases and other cell wall degrading enzymes todigest the plant cell walls and facilitate the access ofhydrolases to the backbone of the wall polymers. Thus,BSG has been effectively used as a carbon source forferuloyl esterase and

This indicated that utilization of abundantly availableand low-cost residues like BSG, as a substrate forenzyme production could be one of the ways whichsubstantially reducesthe enzyme production cost.

Thus,brewer’s spent graincan be used togenerate a wide range of feedstock materials tosupplement current bioethanol production from starchyfeedstock.

Lactic acid production

Lactic acid (2-hydroxy propanoic acid) has found manyapplications in connection with

foods, fermentations,pharmaceuticals and the

chemical industries (Ali

et al.,2009).

Recently,however,there has been an increasing

interest inlactic acid production because it can be usedas a precursor of poly-lactic acid (PLA) production. How-ever, the realization of this potential is dependent onwhether lactic acid can be produced at a low cost whichis competitive on a global scale (Bai etal., 2008).

One of themajor challenges

in the large-scale

production of lactic acid is the cost of the raw

material.The useofexpensive carbon sources such asglucose,

sucrose or starch is not economical

because lactic acid isa

relatively cheap product. Thus, the exploitation of

lessexpensive sources would be beneficial. The agro-industrial

residues are attractive alternatives tosubstitute

these

costly raw materials

(Mussatto et al., 2007a,

2008b).Brewer’s spent grainhas found a prominentpositionas a raw material for lactic acid production in thepresence ofLactobacillus

aids in the release of both xylanase and esterase, whichsolubilize all feruloylated material to ferulic acid. Thus,suggesting that, effective production of ferulic acidrequires the combined action of both xylanase andesterase.Faulds et al. (2002) demonstrated the ability ofcommercialβ-glucanase preparation fromHumicolainsolens

be used toproducexylitol,but BSG has advantage because itrequires no preliminary detoxification steps; but overallproduction is favoured by high initial xylose con-centrations, oxygen limitation, high inoculum